1. Field of the Invention
This invention relates to solar heating systems, and more particularly to closed solar heating systems operating at various flow rates and which are provided with means to drain a solar collector during periods of cold temperature automatically and independently of valve actuation or position.
2. Description of the Prior Art
A perennial problem associated with solar heating systems involves the need to protect the solar collectors exposed to the environmental elements from the effects of cold temperatures. In particular, solar collectors are generally susceptible to breakage during periods of extreme cold when a fluid circulating through the collectors is likely to freeze within the collector unless preventative measures are taken.
One preventative measure suggested in the prior art is to add antifreeze to the collector circulating fluid in order to lower the freezing point of this fluid. Anti-freeze, however, is not only expensive, but also poisonous and therefore represent a serious potential safety hazard. Furthermore, although an anti-freeze mixture does not freeze at lower temperatures, nevertheless the temperature of the mixture does drop to the low ambient temperature. Therefore substantial time is needed to heat the cold anti-freeze mixture to a useable temperature as the solar environment of the collector improves.
It has therefore been desirable to protect solar collectors from fluid freezing by draining the collectors during periods of cold temperature. One such system, as disclosed in U.S. Pat. No. 4,044,754 to Cronin et al., employs drainage valves in piping below the level of the solar collector to drain the collector during periods of low temperature. Also provided at the highest point of the system is a vacuum-braker valve which allows air to enter the water lines to facilitate drainage.
Another solar collector drainage system is disclosed in U.S. Pat. No. 4,027,821 to Hayes et al. wherein a vent line connects the top of the collector to a storage tank and provides an air passageway to the collector from the storage tank to enable drainage of the collector. The vent line is connected to the collector by means of a check-valve and a solenoid actuated valve to initiate drainage of the collector and the vent line.
While the system of the prior art generally provide effective drainage during normal operation, it is seen that they employ mechanically operated valves to initiate drainage. Unfortunately, valves of this type are prone to failure, especially at low temperature when their function is vital to protect the system. In that regard, it is noted that the flow passages of valves tend to be small and are likely to freeze due to condensation or minor back-dripping in the system, and valves are therefore believed to be inherently unreliable drainage components.
It is further noted that "open" drainage systems which utilize valving actuation to admit exterior air into the system during drainage, cannot be easily pressurized due to valve leakage, and are therefore to some extent susceptible to boiling and vaporization at operating temperatures in excess of 212.degree. F. Since vaporization is most likely to occur at the output of the solar collector, a hydraulic imbalance is created between the supply and the return lines to the collector, thereby increasing the load to the system pump. Furthermore, the "open" system introduces fresh oxygen into the system during each system drainage, which contributes to steady corrosion of the flow passages within the system.